This invention relates to zinc sulfide-based electroluminescent phosphors. More specifically, it relates to small-size ZnS:Cu-based blue, blue-green, green and orange electroluminescent phosphors.
There are many uses for electroluminescent (EL) phosphors which have been incorporated into thick film electroluminescent devices. Most commonly, these phosphors have been used for backlighting liquid crystal displays (LCD), automotive dashboard and control switch illumination, emergency egress lighting and mobile phone keypad applications. These phosphors are based on copper-activated zinc sulfide (ZnS:Cu) and may be blue, blue-green, green or orange emitting.
A cross-sectional illustration of a conventional thick-film EL lamp is shown in FIG. 1. The lamp 2 has two dielectric layers 20 and 22. A first conductive material 4, such as aluminum or graphite, coated on a plastic film 12b forms a first electrode of the lamp 2; while a thin layer of a transparent conductive material 6, such as indium tin oxide, coated on a second plastic film 12a forms a second electrode. Sandwiched between the two conductive electrodes 4 and 6 are two layers 20 and 22 of dielectric material 14 which can be, for example, cyanoethyl cellulose or cyanoethyl starch. Adjacent to the first electrode 4 is a layer of dielectric material 14 in which are embedded particles of a ferroelectric material 10, preferably barium titanate. Adjacent to the second electrode 6 is a layer of dielectric material 14 in which are embedded particles of an electroluminescent phosphor 8. The phosphors available for thick-film EL lamps are primarily comprised of zinc sulfide that has been doped with various activators, e.g., Cu, Au, Ag, Mn, Br, I, and Cl. Examples of these phosphors are described in U.S. Pat. Nos. 5,009,808, 5,702,643, 6,090,311, and 5,643,496. Typically, the individual particles of the EL phosphors are encapsulated with an inorganic coating in order improve their resistance to moisture-induced degradation. Examples of such coatings are described in U.S. Pat. Nos. 5,220,243, 5,244,750, 6,309,700, and 6,064,150.
U.S. Pat. No. 4,859,361 to Reilly et al. and International Application No. WO 91/16722 to Faria, describe the process employed in making copper-activated zinc sulfide phosphor particles. First, the phosphor precursor materials, ZnS, a copper source, sulfur, and a chloride flux, are blended and fired to form a hexagonal ZnS material containing copper and chloride ions. Next, the hexagonal ZnS is subjected to low intensity mechanical stresses (by milling and/or mulling) to induce defects into the particles. The stressed material is blended with zinc oxide and copper sulfate and refired at a lower temperature to form an electroluminescent phosphor that has a primarily cubic structure.
There are many characteristics that are commercially desirable, such as brightness, color, and lifetime. Particle size has become an important consideration in the application of these phosphors. As mentioned in the U.S. Pat. No. 5,643,496 to Brese et al., fabrication techniques such as screen printing require smaller particle sizes for thinner phosphor layers. In addition, there has been an interest in using these phosphors in paint or spray-type coatings. Previously, small particle size material has been obtained by controlling the first-step firing time and temperature and/or by screening. However, these processes have low yields.
Thus, it would be advantageous to be able to produce a ZnS:Cu-based EL phosphor that has an average particle size less than 12 micrometers (μm) in diameter and maintains reasonable brightness characteristics. It would also be advantageous to be able to produce these phosphors with higher yields.